1. The Field of the Invention
The present invention relates to a frequency direct-conversion receiving apparatus, and more particularly, to an apparatus for frequency direct conversion reception in a mobile communication terminal and method thereof. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for removing a DC offset and for reducing a leakage power of a local oscillator.
2. Discussion of the Related Art
Generally, a zero intermediate frequency receiver has an intermediate frequency (IF) of 0 Hz and implements a frequency direct conversion system that does not use an intermediate frequency. A frequency direct conversion, also known as a direct conversion system, differs from the widely used super-heterodyne system. Specifically, in a direct conversion system, a high-frequency carrier signal is converted into a low-frequency baseband signal directly without using an intermediate frequency (IF).
FIG. 1 is a block diagram of a zero intermediate frequency receiver. Referring to FIG. 1, a zero intermediate frequency receiver includes an antenna 1 configured to receive radio waves, a band select filter 2 configured to filter the radio waves received via the antenna 1 according to an input reception band, a low noise amplifier (LNA) 3 configured to amplify a received signal with a sufficient gain, an image reject filter configured to filter an image signal working as a noise from the signal amplified by the low noise amplifier 3, a mixer 5 configured to mix the signal provided through the image reject filter 4 and the signal provided from a local oscillator (LO) 7, and a low pass filter 6 through which the output signal of the mixer 5 passes prior to arriving at a baseband processor (not shown).
Where direct conversion is employed, the corresponding configuration is simpler than a super-heterodyne system since there is no intermediate frequency. Various surface acoustic wave (SAW) filters and mixers and the like can therefore be eliminated. Hence, direct conversion is advantageous in achieving a low cost, lightweight, one-chip system. Many efforts have been made to research and develop direct conversion techniques that are applicable to mobile communications in order to improve direct conversion for systems such as the Global System for Mobile communication (GSM).
Direct conversion, which is advantageous in achieving a low cost system with few components, does have some drawbacks when compared to systems using an intermediate frequency (IF). For example, applications using direct conversion may be limited to standard frequencies and systems. Also, direct conversion frequently generates an undesirable DC offset. This undesirable generation of a DC offset has several causes. First, an undesirable DC offset can occur in the output of a mixer due to self-mixing. Self-mixing occurs when a leaked signal of a local oscillator is inadvertently input into a mixer via an unintended path. Second, an undesirable DC offset can occur when a leaked signal of a local oscillator is inadvertently input into a low noise amplifier. When this occurs, the low noise amplifier produces a signal that includes a DC offset. Third, an undesirable DC offset can occur when a radio wave transmitted via an antenna is reflected back, and a noise is generated when the radio wave is converted to a baseband signal. Also, the leaked signal of a local oscillator (LO) is generated from bonding wire radiation, magnetic coupling, ground instability and the like.
Since direct conversion converts a radio signal directly into a baseband signal, gain and filtering are mostly carried out on a frequency band between DC and signal bandwidth. In doing so, a unique DC offset of a signal path is extended to degrade the operational range of a circuit. Hence, a DC offset, which is a disadvantage of a zero intermediate frequency receiver, needs to be compensated for in order to achieve optimal radio performance.